1. Field of the Invention
The present invention generally relates to an optical scanning method, an optical scanner and an image forming apparatus, and more particularly to an optical scanning method and an optical scanner for writing latent images by radiating optical beams on scanned surfaces of a plurality of linearly arranged image carrying members, and an image forming apparatus such as a copier, a printer and a plotter that can form a multi-color image by developing the latent images with distinct color developers and then sequentially transferring the developed color images to a transferred member.
2. Description of the Related Art
In a conventional tandem type color image forming apparatus, optical beams emitted from a plurality of illuminants are radiated to four linearly arranged image carrying members such as photosensitive drums in order to write latent images thereon. The latent images formed on the image carrying members are developed to visualize the latent images by using distinct color developers, typically, a yellow toner, a magenta toner, a cyan toner and a black toner. Then, a transferred member such as a recorded paper is carried on a transfer belt to each transferring part of the image carrying members, and the individual color images are sequentially superposed on the transferred member. The resulting color image on the transferred member is fixed, and it is possible to produce a multi-color image.
In such a conventional tandem type color image forming apparatus, an optical scanner, such as an optical writing apparatus, is prepared for each of the image carrying members, and the optical writing apparatus writes a latent image on the corresponding image carrying member. However, the optical writing apparatus is relatively expensive because the optical writing apparatus contains an optical deflector formed of a polygon mirror and a drive motor for driving the optical deflector. For this reason, components and assembly costs of the conventional tandem type color image forming apparatus can be problematic, as it is necessary to provide a plurality of optical writing apparatuses corresponding to the plurality of image carrying members. In addition, it is necessary to provide an adequate installation space in the image forming apparatus to accommodate the optical writing apparatuses each of which includes an optical deflector. As a result, it is impossible to avoid a size increase in an image forming apparatus in which it is desired to include such optical writing apparatuses.
Furthermore, although a tandem type color image forming apparatus is capable of forming a color image, the occasion in offices to produce monochrome manuscripts is greater than that of color manuscripts. As the tandem type color image forming apparatus is required to produce more full-color manuscripts at higher speeds, the tandem type color image forming apparatus has more significant problems, including the following:                1. a complicated mechanism for superposing four colors,        2. a cost increase of motors and drive parts for driving photosensitive members,        3. a short life span of the motors and the drive parts for driving the photosensitive members.        
In order to meet such office use, conventional color image forming apparatuses are designed to achieve higher productivity in a monochrome mode than in a full-color mode; that is, to operate in the monochrome mode at higher line speed than in the full-color mode. Such color image forming apparatuses can offer monochrome manuscripts at higher speed than full-color manuscripts; that is, the color image forming apparatuses can form more images in the monochrome mode per unit of time than in the full-color mode.
On the other hand, there is a color image forming apparatus that allows a user to switch between a quality priority mode and a speed priority mode. For instance, the color image forming apparatus produces an image at a resolution of 1200 dpi in the quality priority mode and at a resolution of 600 dpi in the speed priority mode. In the quality priority mode, the image forming apparatus writes an image at a higher write density under a constraint of lower line speed so that a high-quality manuscript can be obtained, albeit at the cost of a slower operating speed. In contrast, in the speed priority mode, the image forming apparatus writes an image at higher line speed under a constraint of moderate image quality so that high-speed operations can be achieved, albeit at the cost of a lower resolution image quality.
In the above-mentioned color image forming apparatus, when a user wants to obtain more monochrome manuscripts in the speed priority mode than in the quality priority mode, a user is allowed to select the operation mode from the quality priority mode and the speed priority mode by switching the pixel density. In the conventional color image forming apparatus, two beams for black (BK) are prepared therein together with a pitch switching mechanism, and one beam for each of yellow (Y), magenta (M) and cyan (C) is provided therein. Then, there are four mode combinations: a monochrome quality priority mode, a monochrome speed priority mode, a color quality priority mode, and a color speed priority mode. In the monochrome quality priority (1200 dpi) mode, the color image forming apparatus operates two BK beams at a pitch of 1200 dpi with respect to the sub-scanning direction at low line speed. In the monochrome speed priority (600 dpi) mode, the color image forming apparatus operates the two BK beams at a pitch of 600 dpi with respect to the sub-scanning direction at high line speed. In the color quality priority (1200 dpi) mode, the color image forming apparatus operates color beams and one of the two BK beams, each of which writes an image at the pitch of 1200 dpi with respect to the sub-scanning direction at low line speed. At this time, only one of the two BK beams is switched ON. In the color speed priority (600 dpi) mode, the color image forming apparatus operates the color beams and one of the two BK beams, each of which writes an image at the pitch of 600 dpi with respect to the sub-scanning direction at high line speed.
According to the above-mentioned color image forming apparatus, when resist positions of four colors (BK, C, M, Y) are adjusted with respect to the main scanning direction and the sub-scanning direction (only one beam is used for BK), it is necessary to properly set a pixel density switching position of BK as either 600 dpi or 1200 dpi. If the pixel density switching position is not properly adjusted, there is a probability that a produced full-color image has a color difference due to misalignment of the BK write position as shown in FIGS. 1A and 1B.
FIGS. 1A and 1B show dot positions of optical spots for two-beam writing under two pixel densities of 1200 dpi and 600 dpi. FIG. 1A shows dot positions of a first beam and a second beam at a resolution of 1200 dpi, and FIG. 1B shows dot positions of a first beam and a second beam at a resolution of 600 dpi.
As shown in FIG. 1A, when an image is written at the resolution of 1200 dpi with respect to the sub-scanning direction, a pitch of 21 μm (=25.4 mm/1200) between adjacent optical spots is obtained. As shown in FIG. 1B, when an image is written at the resolution of 600 dpi with respect to the sub-scanning direction, a pitch of 42 μm (=25.4 mm/600) between adjacent optical spots is obtained. As seen in FIGS. 1A and 1B, a dot position of an optical spot has a difference L of 10.5 μm between the two resolutions, as computed from the following formula:L=(42 μm−21 μm)/2
FIG. 2 shows the difference of dot positions with respect to the sub-scanning direction between the two resolutions. When one of the BK beams is used in the full color modes, there is a probability that a color difference between BK and another color (cyan in FIG. 2) may occur with respect to the sub-scanning direction if beam pitch positions are not properly adjusted for alternation between the two resolutions of 1200 dpi and 600 dpi. This color difference is caused by the narrowed beam pitch between BK and the other color by the difference L.
On the other hand, FIGS. 3A through 3C show a difference of dot positions with respect to the main scanning direction between the two resolutions. As shown in FIG. 3A, full-color adjustment for properly producing full-color images is performed for the first beam with respect to the main scanning direction. In fact, however, if the second beam, which is not adjusted, is used to form the full-color images, a color difference arises between the second beam and the other color beams with respect to the main scanning direction, as is shown in FIG. 3B. As previously mentioned, this color difference is caused by the difference of dot positions of BK beams as shown in FIG. 3C. As used herein, the phrase “full-color adjustment” means to correct color differences caused at shipment and during use. Japanese Laid-Open Patent Application No. 11-301032 discloses an adjustment technique for correcting such color differences.